Kai is the Great Child of the 3rd World, Trius, and his Spirit's name is Bakuzan (バクザン). He is a martial artist specializing in fighting with staffs and a disciple of the mysterious Master Funen, who was cursed by the Needle Tower.

Kai has admired En since he was little, when En saved his village from a giant Troublemaking Monster called Taurus. He forms a trio with Baku and Pink where the three go on a journey to the Needle Tower. Kai also has a giant purple flying ray named Manta as a Monster-Guide.